Heretofore, difficulties were experienced due to atmospheric scintillation effects, which destroy the spatial coherence of the wavefront across a detected area. Differential atmospheric scintillation causes the intensities of laser and "point source" backgrounds to differ from detector to detector. While this is not a serious limitation in high flying aircraft, where atmospheric scintillation is spatially well correlated, it is an important limitation for low flying aircraft or ground systems. At ground level the spatial correlation distance of sunlight is of the order of a mere seven millimeters. In an unequal path interferometer system such as, for example, a Fizeau or a Fabry-Perot stepped-etalon system reliance is made on achieving coherent signal detection and absolute background and background scan noise (incoherent signal) rejection by means of balanced detectors and signal cancellation (subtraction) techniques.
Several proposals to mitigate this problem are set forth in the aforesaid U.S. Pat. No. 4,309,108. One proposal was to make all the detectors view the scene through a common aperture so that the light intensities incident on all parts of the etalon were identical. The means for equalizing the radiation intensity between the detectors, according to one form set forth in said patent resided in the provision of a beam-splitter wherein the radiation entering the beam-splitter was divided so that one portion passed through the beam dividing surface to a first one of the regions of the etalon and a second portion was reflected from the beam dividing surface to a second one of the regions of the etalon. Thus, a common aperture means was provided for directing radiation to all portions of the etalon. Preferably, according to said patent, the beam-splitter had a plurality of very small dots on the beam dividing surface, which were highly reflective and the spaces therebetween were highly transmitting.
In another form according to said prior art patent, equalization of the radiation intensity between the detectors was effected by means of providing an etalon having a plurality of regions of different thicknesses wherein the regions were in a form of a plurality of interdigitated steps. A plurality of detectors were provided corresponding in number to the etalon regions, said detectors having a plurality of interdigitated elements corresponding to the interdigitated steps of the etalon, that were adjacent or in contact therewith. The widths of the detector elements were made as small as possible.
In summary, where an optical signature is observed in a turbulent medium such as the atmosphere and where observations in different wavelength regions, or amplitudes, etc., are to be compared simultaneously, the spatial intensity fluctuations due to the turbulence or scintillation can not be cancelled or subtracted unless the simultaneous observations can be performed over a sampling distance which is small compared to the spatial fluctuation length. It is an object of the present invention to provide means for doing so which extends the lower size limit significantly beyond that achievable with prior art detector arrays, and which allows for the simultaneous observation of four or more wavelengths with a single chip detector which is essential to exactly match comparison circuits.